Marathon Swim Fueling Strategies: Lessons From the Water

Feed break on the Santa Barbara Channel

I didn’t get certified in nutrition and sports nutrition because I wanted more letters after my name.

I did it because I got sick.

On my swim around Coronado Island, I started vomiting about five hours in. My fueling plan was built around slow carbs, because my stomach is notoriously wonky. It was supposed to be “steady,” “gentle,” “easy on the stomach.” For me, it wasn’t.

For my Anacapa swim, I adjusted. I switched to fast carbs, cyclic dextrin, hoping faster absorption would solve the problem. Instead, that swim turned into an 11-plus hour grind thanks to the current, and for the last two hours, I had diarrhea.

Same athlete. Two different fueling strategies. Same outcome: my gut couldn’t keep up.

I wasn’t entirely surprised. As a runner and long-course triathlete, I’ve always had a sensitive stomach. But what I didn’t understand at the time is that marathon swimming isn’t just another endurance sport with a different outfit.

It’s its own physiological problem.

The Research Gap Nobody’s Talking About

Here’s the uncomfortable truth: there is a mountain of research on fueling for runners and cyclists. For marathon swimmers? Almost nothing.

And the differences aren’t minor.

Cold water changes everything. We know endurance exercise diverts blood away from the gut, slowing digestion, that’s well established. But add cold water to the equation and you’ve got peripheral vasoconstriction on top of it, meaning your body is also working overtime to preserve core temperature. That means even less blood flow to your digestive system. In practice, this likely explains why some swimmers report nausea, delayed gastric emptying, and sudden, dramatic rejection of feeds hours into a swim. We don’t have clean studies isolating this in marathon swimmers yet, but anecdotally it shows up over and over again.

You’re horizontal, and gravity matters. Cyclists are upright. Runners are upright. Swimmers are not. That changes how fluids sit in your stomach, how quickly they empty, and how pressure builds. If you’ve ever felt fuel sloshing mid-swim, you’ve felt this firsthand. Without gravity assisting gastric emptying, you’re relying on internal pressure and motility, both of which are already compromised during long efforts.

You can’t sip. You bolus. In most endurance sports, fueling is continuous or frequent. In marathon swimming, you’re feeding roughly every 30 minutes, which means you’re not sipping, you’re dumping. A full bolus of fluid and calories hits your stomach all at once. Do that repeatedly over 8, 10, 12 hours and you’ve created a slow-building recipe for gastric overload, nausea, and a widening gap between what you’re taking in and what you’re actually absorbing.

Kayaking for a swimmer on the Catalina Channel in July 2026

What I’ve Seen From the Kayak

I have kayaked for nearly 100 marathon and training swims. Patterns emerge quickly when you watch that many swims.

From the nearly 100 marathon and training swims I've observed from the kayak, I'd estimate that roughly one in ten swimmers vomit at some point during a long swim. Many more experience bloating, nausea, or gradually begin skipping feeds as the hours wear on. The longer the swim, the less likely the athlete is to stick to their plan.

Here’s what’s particularly interesting: faster swimmers tend to have less fueling problems.

Swimmers who complete a 20-mile channel crossing in under 10 hours often report fewer issues, but that doesn’t necessarily mean they’re “better at nutrition.” It may mean they’re working at a higher relative intensity, oxidizing fuel more efficiently, and not accumulating unprocessed carbohydrates in the gut. Their system is actually using what they’re taking in.

Slower, or more accurately, average paced swimmers may be taking in similar amounts while burning less per hour, and that accumulation is exactly where things go sideways.

So What Do We Actually Do About It?

There's no perfect formula. But there are better approaches.

Train your gut, not just your stroke. This is the most overlooked skill in marathon swimming. Fueling isn't something you figure out on event day. It's something you train, the same way you train your catch or your kick. That means using your exact fuels during training swims, even short ones. Start practicing your feed schedule at 30 minute intervals, and building your body's tolerance for volume, not just calories.

Here's the part most swimmers skip, and I'll be the first to raise my hand. Sidney suggested I set a timer at my desk and every 30 minutes, using water, drink the same amount I would during a feed. I did it for a couple of hours on a couple of occasions and then promptly moved on with my life. Standing here on the other side of two marathon swims, I really wish I had taken that advice more seriously. You're not just training digestion. You're training tolerance. Because when things fall apart at hour six, it's usually not a fitness problem. It's a gut problem.

Find your burn rate. Generic advice, such as "60 to 90 grams of carbs per hour" or "stick to X calories," is a starting point, not a prescription. Someone like Andy Donaldson is operating at a completely different metabolic demand than most swimmers. His intake needs are higher, and his system is trained to handle it. Your job is to figure out how much you actually burn and how much you can actually absorb. Those are not always the same number. The gap between them is where problems start.

And beyond quantity: what type of fuel works for you? Fast carbs? Slow carbs? A blend with some protein? Cold feeds or warm? These are worth experimenting with long before you're treading water off Catalina at 2 a.m.

Where This Leaves Us

Even experts like Peter Attia, who completed the Catalina Channel Swim, have talked extensively about the complexity of fueling in extreme endurance events, but even those insights are largely borrowed from running and cycling.

Marathon swimming sits in a genuine blind spot. Cold exposure. Horizontal movement. Intermittent feeding. Ultra-long durations. It's a different system, and it doesn't get nearly enough attention.

That's exactly what we're here to dig into, and we mean that literally. Sidney and I are working with some other experts to do a deep dive into marathon swim fueling, and we want this community to be part of it. Attached is a link to the initial intake form for the Marathon Swim Fueling Registry [LINK HERE]. Your experiences, your fueling disasters, your hard-won wins, all of it helps us build something more useful than generic advice borrowed from runners and cyclists.

Every finding from this study will be shared freely through articles, presentations, and community resources. No paywalls, no gimmicks. Just real data from real swimmers, for real swimmers.

Because fueling in marathon swimming isn't just about fast carbs versus slow carbs, or hitting a calorie target, or getting your electrolytes right. It's about what your body can actually process while moving through cold water, horizontally, for a very long time.

And that system breaks down, not because you're weak, and not because your plan was bad, but because you're asking your body to do something it's not fully designed, or fully studied, to do yet.

So we study it. We share it. And we train it.

Just like everything else.